Backlinks should be build to imitate a natural growth of a site. If we translate that into backlink structure, we can understand that monotonous and “too optimal” structures look unnatural.

Lets try to see that by looking at two examples:

Do you see something unusual about this chart? Is it harder to understand? Well it is because it is an obvious search engine manipulation (a bit extreme, but you get the point!). Having only PR5 sites, all dofollow is as unnatural as it gets and will turn on the red light with most search engines.

How should a proper backlink structure look like?

See the difference? You can actually see variety! Although you should decide how to structure the percentages of certain backlink (10% blog comments, 15% do-follow links etc) yourself. Just like in keyword density, there is no correct answer you can have about the percentages. But you can have a wrong answer: Anchor text over-optimization sprinkled with a bad backlink variety.